PDGF is the major mitogen in serum for mesenchymal-derived cells. PDGF is stored in platelet .alpha.-granules and released locally during platelet activation when blood vessels are injured. PDGF is a potent chemoattractant for monocytes and neutrophils and for fibroblasts and smooth muscle cells. These activities make PDGF an important component in tissue repair processes.
Purified native PDGF is a glycoprotein of approximately 30,000 daltons and is composed of two disulfide-linked chains. There are two types of chains, designated A and B. Whether native PDGF is a heterodimer, a mixture of homodimers or a mixture of heterodimer and homodimer(s) is not known, but the dimer structure is functionally important, since reduction irreversibly destroys the biological activity of PDGF.
The B-chain is derived by proteolytic processing of a 241 amino acid precursor. The B-chain precursor is encoded by the c-sis gene, the cellular counterpart to the transforming gene v-sis of simian sarcoma virus (SSV). cDNA encoding the B-chain has been reported previously in Nature (1985) 316:748-750. There is homology between the B-chain and the transforming protein v-sis. The cloning and expression of the v-sis gene is described in EPA 85112852.0 (Publication no. 0177957). Studies of v-sis indicate that B-chain homodimers have mitogenic activity. Also, sequencing of porcine PDGF has revealed that it contains only one type of chain, corresponding to human B-chain (EMBO J (1984) 3:2963-2967).
Johnsson, A., et al, EMBO J (1984) 3:921-928 describe a partial amino acid sequence for PDGF A-chain. See also Nature (1983) 304: 35-39 and Science (1983) 221: 275-277. Heldin, C. H. et al, Nature (1986) 319:511-514 describes an osteosarcoma-derived growth factor (ODGF) that is structurally related to putative PDGF A-chain homodimer. The studies of ODGF suggest that PDGF A-chain homodimer would exhibit biological activity.